1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus for forming an image having an arbitrary line width with a plurality of image lines.
2. Description of the Related Art
Image recording and image erasing on a thermoreversible recording medium have been carried out so far by a contact method in which a heating source is brought into contact with a recording medium to heat the thermoreversible recording medium. As the heating source, in the case of image recording, a thermal head is generally used, and in the case of image erasing, a heat roller, a ceramic heater or the like is generally used.
Such a contact image processing method has advantage in that, if a thermoreversible recording medium is a flexible material (e.g., a film, and paper sheet), an image can be uniformly recorded and erased by evenly pressing a heating source against a thermoreversible recording medium with use of a platen etc., and an image recording device and an image erasing device can be manufactured at low costs by using components of a conventional thermosensitive printer. However, when an RF-ID tag as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2004-265247 and Japanese Patent (JP-B) No. 3998193 is embedded in a thermoreversible recording medium, the thermoreversible recording medium needs to be thickened and the flexibility thereof degrades, and thus high pressure is required for uniformly pressing a heat source against the thermoreversible recording medium. In addition, in the contact image processing method, a surface of the thermoreversible recording medium is scraped due to repetitive printing and erasure and irregularities are formed therein, and some parts are not in contact with a heating source (e.g., thermal head, and hot stamp). Thus, the thermoreversible recording medium may not be uniformly heated, causing degradation in image density and erasing failure (see Japanese Patent (JP-B) No. 3161199 and Japanese Patent Application Laid-Open (JP-A) No. 09-30118).
In view of the fact that RF-ID tag enables reading and rewriting of memory information from some distance away from a thermoreversible recording medium in a non-contact manner, a demand arises for thermoreversible recording media as well. The demand is that an image be rewritten on such a thermoreversible recording medium from some distance away from the thermoreversible recording medium. To respond to the demand, a method using a laser is proposed as a method of forming and erasing each image on a thermoreversible recording medium from some distance away from the thermoreversible recording medium when there are irregularities on the surface thereof (see JP-A No. 2000-136022). It is the method by which non-contact recording is performed by using thermoreversible recording media on shipping containers used for physical distribution lines. Writing is performed by using a laser and erasing is performed by using a hot air, heated water, infrared heater, etc, but not by using a laser.
As such a recording method using a laser, a laser-recording device (laser maker) is proposed by which a thermoreversible recording medium is irradiated with a high-power laser light to control the irradiation position. A thermoreversible recording medium is irradiated with a laser light using the laser marker, and a photothermal conversion material in the thermoreversible recording medium absorbs light so as to convert it into heat, which can record and erase an image. An image recording and erasing method using a laser has been proposed, wherein a recording medium including a leuco dye, a reversible developer and various photothermal conversion materials in combination is used, and recording is performed thereon using a near infrared laser light (see JP-A No 11-151856).
However, in such a laser recording method, when an information-read code such as a two-dimensional code (e.g., character, bar code, and QR code) is recorded, but if an image having a predetermined line width is not precisely formed, the code may not be satisfactorily read through a machine, although the recorded image appears cleanly written visually. Also, when lines are written on a recording medium in an overlapped manner in an attempt to write a line having a width greater than the beam diameter of a laser beam used, the thermoreversible recording medium is excessively heated due to accumulation of heat, causing degradation in the repetitive durability of the thermoreversible recording medium.
JP-A No. 2008-213439 proposes a method for uniformly heating a recording medium, and JP-A No. 2008-62506 proposes a method for forming an image excellent in readability. However, the above methods have drawbacks in that it is impossible to precisely form an image having a predetermined line width, and the repetitive durability of recording media degrades.
As a printing method of two-dimensional codes, JP-A No. 2001-147985 proposes a method in which each cell is scanned in a spiral manner with a laser beam to print a code. In addition, JP-A No. 2006-255718 proposes a method in which the scanning position of a laser beam is corrected to obtain a predetermined line width. However, the above methods have a drawback in that the repetitive durability of recording media is poor, although it is possible to precisely form a predetermined line width.
When an image of two-dimensional codes (e.g., characters, bar codes, and QR codes) in various line thickness and various sizes is formed by laser marking, it is necessary to precisely form the image having a predetermined line width. Particularly in recording of bar codes, the recording accuracy influences the readability of the bar codes, and thus there is a need to precisely form various line widths. Further, when an image is formed on a rewritable thermoreversible recording medium and if an excessive amount of energy is applied thereto, the thermoreversible recording medium is physically damaged, causing degradation in the repetitive durability. Thus, to form an image having a predetermined line width, it is also required to uniformly apply energy to the thermoreversible recording medium.
Although the beam diameter of a laser beam irradiated to a thermoreversible recording medium is constant, the beam has a light intensity distribution, and thus the line width of an image can be changed by altering the irradiation power or the scanning speed of the laser beam so as to control the irradiation energy applied to the thermoreversible recording medium. However, when the irradiation energy is increased, unfavorably, the thermoreversible recording medium is physically damaged, causing degradation in repetitive durability, although the image is formed with a broader light width. On the other hand, when the irradiation energy is reduced to prevent degradation of the repetitive durability of the thermoreversible recording medium, the image is formed with a narrower line width, however, the contrast (density) of the formed lines decreases, causing degradation in image quality.